Standardize and expose movements by word in the REPL

Make all movement by word skip initial whitespace. Define and expose deletion by word to the right, as well.

@nolta and I were working on this at the same time (e27771df954f44551eba06fdc7d1a9f6e4f12ab8 and https://github.com/mbauman/julia/compare/JuliaLang:master...mbauman:REPL-word-bindings).  This takes the best of both.